Design of an off-axis HOE light concentrator to focus light from multiple directions in a plane

The attractive and multifunctional properties of holographic optical elements (HOEs) offer an opportunity to create compact optical systems for a wide range of applications. In this paper, we propose a novel method of focusing light onto a small region of space by an off-axis HOE that is illuminated from multiple directions in a plane. An off-axis spherical wave generated from a microscopic objective (0.65 NA) is combined with sequentially illuminated five reference waves at the recording plane. A multi-exposure technique is adopted for recording. A comparison between single-exposure and multi-exposure technique is also presented. The design parameters, performance and possible applications of these optical devices are described.